Trees through the roof and in the courtyard: what is involved

We are increasingly seeing this in deliveries and projects: trees as part of a building. They are usually situated in a courtyard, patio or covered outdoor space, and therefore largely outdoors. The building is designed around the tree, with the trunk passing through a roof, canopy or terrace deck. Sometimes construction takes place around an existing mature tree. In other cases, the large tree is planted first and the building follows afterwards.

There is a clear trend behind this demand. Cities are becoming denser, and designers want to incorporate mature greenery within the contours of a building as part of how a place is used. A courtyard with a real tree turns an enclosed building into a place where people enjoy spending time. This requires a tree that looks mature from day one, and therefore a large planting size at the time of planting.

The architecture and associated renders for these developments often look grand and green. In such a render, the tree is always fully grown, and the contrast between the building and the greenery beside it is precisely why a large planting size is requested.

This difference between indoors and outdoors determines almost everything that follows. A tree that stands largely outside grows under normal outdoor conditions. Very different conditions apply in an enclosed, heated space. Below are the factors that are decisive during the design phase.

Passing the trunk through the roof or terrace

The defining detail is where the trunk passes through the roof or terrace deck. Coordinate this with the structural engineer at an early stage. The trunk grows thicker every year, so make the opening sufficiently large or allow it to expand with the tree. And as soon as the crown is exposed, the tree moves in the wind: the edge must not rub against the bark, and water must be drained away along the trunk.

Such a tree often falls outside standard nursery forms. Consider a clear stem of more than 4 metres with a crown of 3 metres above it.

Root space and soil

If the tree is already there, protect its roots. A changed ground level, compaction caused by machinery or paving over the roots causes damage that may only become visible years later. Keep this area clear and uncompacted.

If a new tree is planted in a planting bed or planter, the rootable volume sets the upper limit for its eventual size: if it is too limited, the tree dries out more quickly. Once the project has been completed, this space is fixed. A rule of thumb allows approximately 1 m³ for every year the tree is expected to remain in place, so allow for several tens of cubic metres. Also provide a dedicated water supply and drainage.

Light

A tree crown needs light, including in a courtyard. With an open patio or a roof with a large opening, this is rarely a problem. If the walls are high and opaque, the opening above the tree becomes especially important. The further the tree is from that opening, the less daylight the crown receives. Pay particular attention to winter, when there is the least daylight. Position the tree as close beneath the opening as possible and, where light is limited, choose a species that tolerates this well.

Reflection of sunlight from building façades can, however, create a magnifying-glass effect, causing leaves, the trunk or the litter layer to scorch.

Trees in a heated indoor space

If the tree stands largely outdoors, in a courtyard or beneath an open roof, it lives under normal outdoor conditions and the usual range is suitable. This only changes in a fully enclosed, heated space.

Deciduous broadleaved trees from our climate become dormant in winter and need a long period of cold to remain dormant and flush normally in spring. If they do not receive this cold, as in a heated space, the tree flushes irregularly and does not survive. For such places, several evergreen species from warmer regions are suitable, as they do not require cold winter dormancy and tolerate dry air, such as Olea europaea and Quercus ilex. They like warmth, but also need a light position close to the glass. Warm and too dark is unsuitable for every tree.

Installing large trees during construction

Mature greenery requires large trees, and these must not only fit the design but also be able to reach their position in practical terms. The load-bearing capacity of the structure, the delivery route and crane operations often mean that the tree is installed early in the construction process, while the space is still open. The larger the tree, the earlier this must be incorporated into the planning.

Where we become involved

Most of the work takes place during the design phase, long before delivery. This is when decisions on the location, trunk opening and root space are fixed. Our advisers contribute thoughts on species, size and location, working with the architect and structural engineer to establish what the tree needs at that location in order to establish, from the first sketch to the day it is planted. With the enormous choice of large specimens grown in our own nurseries, there is almost always a tree that suits the location.

Do you have a project in which a tree plays a role through or within the building? Put it to us at an early stage, and we will consider what is feasible.